Phagocytosis

Phagocytosis is the process by which a cell engulfs and consumes other cells or solid particles. In single-celled organisms such as the amoeba, phagocytosis is a method of feeding. In higher organisms, the process is a vital part of the immune system and is carried out by specialized cells called macrophages and certain white blood cells. It also helps clear dead and dying cells, which helps keep tissues healthy. These cells, known as phagocytes, engulf and digest invading organisms such as bacteria, viruses, or other foreign bodies. The term phagocytosis comes from the Greek words phagein, meaning “to eat,” and cytos, meaning “cell.”

Background

Cells are covered by an outer plasma membrane that protects the interior and regulates the substances that enter the cell. Smaller molecules can easily pass through the cell membrane, but to take in larger molecules, cells use a process called endocytosis. During endocytosis, the cellular membrane moves inward, curving around the molecule and surrounding it. Phagocytosis is a type of endocytosis in which a cell ingests solid particles. Pinocytosis, or cellular drinking, occurs when the cell consumes liquids.

By the nineteenth century, scientists had identified white blood cells and observed them in the process of surrounding bacteria. The prevailing idea at the time, however, was that these cells carried the foreign invaders and spread them throughout the body. In the 1880s, Russian microbiologist Élie Metchnikoff observed the action of “wandering” cells in transparent starfish larvae and noticed that the cells seemed to clump near the site of an invading organism, as if they were attacking it. He realized that the cells were consuming the foreign bodies, not spreading them. He coined the term phagocytosis to describe his discovery, for which he shared the Nobel Prize in 1908.

Overview

Microscopic organisms that consist of a single cell sustain themselves through the process of phagotrophic nutrition. When an amoeba, for example, detects the presence of a bacterium, it crawls toward its target. When it makes contact, the amoeba sends out “arms” called pseudopodia formed from its cell membrane. Some single-celled organisms use hair-like structures called cilia to help capture food particles. The pseudopodia surround the bacterium and encase it in an internal cavity called a vesicle. Before the bacterium can reproduce, the amoeba releases other vesicles packed with digestive enzymes, proteins that produce a chemical reaction in organic substances. These vesicles attach themselves to the food, break down the bacterium, and absorb the nutrients.

In the human body, phagocytosis occurs in many cells, but the most common are white blood cells found in the immune system. These cells make up less than 1 percent of the body’s blood and act as natural infection fighters. Of the five types of white blood cells, monocytes and neutrophils are the most accomplished phagocytes. Other specialized phagocytes include dendritic cells, which help activate immune responses. Monocytes are the largest white blood cells and develop in the bone marrow, a soft substance in the center of the bones. After monocytes are released into the bloodstream, they travel to the site of an infection and enter the body’s tissue. They then develop into specialized infection-devouring cells called macrophages. The large size of the cells makes it easier for them to consume foreign invaders.

Since macrophages must come into contact with an infectious organism before consuming it, the body must first find a way to let the macrophage know which object to target. One way is for other white blood cells to produce proteins called antibodies that attach themselves to certain molecular patterns on the surface of the invading cell. These antibodies act as a homing beacon for the macrophages, marking a particular cell for destruction. Some macrophages have specialized receptors on their surface designed to search for specific antibodies and bind with them. Others have receptors designed to seek out and bind with certain molecules produced by the bacteria itself.

When the receptors on the surface of the macrophage bind with the target, the macrophage begins drawing the cell inward, encircling it, and closing around it. The target becomes enclosed inside an internal bubble called a phagosome. The phagosome also helps the cell examine the trapped material and prepare an immune response. This structure then fuses with an enzyme-filled bubble inside the cell called a lysosome. Once formed, the merged structure, known as a phagolysosome, reduces its internal pH level. A pH level is a measure of acidity; the lower the pH level, the more acidic the environment. This kills or neutralizes the target before it has a chance to reproduce, allowing the macrophage’s enzymes to break it down for absorption. The resulting waste product is later expelled from the macrophage.

Some pathogens have the ability to avoid destruction after being engulfed by a phagocyte. When a macrophage becomes infected by a virus, the body’s other phagocytes will then mark it as a target.

Neutrophils are the most numerous white blood cells, making up about 40 to 70 percent of the body’s total white blood cells. The first line of defense to arrive at the site of an infection, neutrophils act as natural assassins (rapid-response immune cells), using several methods to kill their targets. They are also produced in the bone marrow and live for several hours to a few days. Their primary method of attack is similar to the macrophage; they engulf an invader and digest it through enzymes released from granules, or grain-like shapes, within their structures. Neutrophils can also emit a burst of highly reactive oxygen molecules that can damage and destroy invading cells. Neutrophils and other phagocytes can also kill their engulfed prey by releasing antimicrobial proteins specifically designed to destroy bacteria or producing binding proteins that interfere with a bacterium’s ability to reproduce.

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